Tone is the author’s attitude toward the subject. In this case, the attitude toward the supernatural events of the story.

The dominant spirit, however, that haunts this enchanted region, and seems to be commander-in-chief of all the powers of the air, is the apparition of a figure on horseback, without a head.

Although there is certainly a gloomy and foreboding sense to these lines, there is also a serious note. The narrator changes, but there is always a sense of gravity, rather than just humor, to the telling of the tale. Since foreshadowing is used, as well as this "I am going to tell you a story" framing, we know that something bad will eventually happen.